1. Field of the Invention
The present invention relates to an apparatus and method for accurately demodulating a deeply angle-modulated signal.
2. Description of the Related Art
An apparatus and method of demodulating an angle-modulated wave with using analog signal processing has been employed. In recent years, a method of demodulating an angle-modulated wave with using digital signal processing has been widely proposed according as a digital signal processing technique develops.
A conventional angle demodulator, with digital signal processing, is composed of an FM demodulator in which an FM signal is demodulated by applying the FM signal and a signal whose phase has shifted ninety degrees with respect to that of the FM signal.
The FM demodulator demodulates the FM signal based on equation (1) where X represents the FM signal to be demodulated and Y represents a signal whose phase advances ninety degrees with respect to the FM signal, and F represents a result of the FM signal being demodulated.
F=(d/dt)arctan(X/Y)xe2x80x83xe2x80x83(1)
As illustrated in FIG. 5, the FM demodulator comprises, for example, an A/D converter 101, a delay compensator 102, a ninety-degree phase shifter 103, a phase angle calculator 104, a differentiator 105 and a D/A converter 106.
In this type of FM demodulator, the FM signal X is input to the A/D converter 101 so as to be converted into a digital signal. This digital signal is input both to the delay compensator 102 and to the ninety-degree phase shifter 103.
The ninety-degree phase shifter 103 generates a digital signal representing a signal Y whose phase shifts ninety degrees with respect to phase of the input digital signal, and outputs the phase-advanced digital signal to the phase angle calculator 104. The delay compensator 102 delays the input digital signal for a time period of a delay which occurs between an input and an output of the ninety-degree phase shifter 103. The delayed digital signal is output to the phase angle calculator 104.
The phase angle calculator 104 divides a value of the delayed digital signal input from the delay compensator 102 by a value of the phase-advanced digital signal input from the ninety-degree phase shifter 103. Further, the phase angle calculator 104 obtains an arctangent of the obtained quotient so as to send the digital signal representing the obtained arctangent to the differentiator 105.
The differentiator 105 obtains a difference between two digital signals which are input continuously (in other words, a difference between values of the two signal segments which are provided continuously), and outputs a digital signal representing the obtained difference to the D/A converter 106. The digital signal from the differentiator 105 represents a value of the signal F as a resultant signal of the FM signal X being demodulated.
The D/A converter 106 generates the signal F by converting the digital signal input from the differentiator 105 into an analog value.
In the FM demodulator shown in FIG. 5, the phase angle calculator 104 selects a value as a value of the arctangent (i.e., as a principal value) from a range of predetermined continuous 2xcfx80 radians within an infinite number of sustainable values.
In the case where the angle-modulated signal is deeply modulated and the phase of the angle-modulated signal shifts greater than 2xcfx80 radians, the phase angle calculator 104 tends to select an inappropriate value, such as a value which is 2xcfx80 radians greater than a value of a true phase angle, or a value which is 2xcfx80 radians less than a value of a true phase angle. The FM demodulator illustrated in FIG. 5 thus can not demodulate in an appropriate manner an angle-modulated signal having the phase shifting of greater than 2xcfx80 radians.
The present invention has been made in consideration of the above, and an object thereof is to provide an apparatus and method for accurately demodulating an angle-modulated signal, even in a case where a phase of the angle-modulated signal shifts greater than 2xcfx80 radians.
In order to achieve the above-described object, according to the first aspect of the present invention, there is provided an angle demodulator comprising:
a phase shifter (81) which inputs a digital angle-modulated signal (SI) thereto and generates a first ninety-degree phase shifted signal (SQ) representing a signal whose phase is substantially ninety degrees out of phase with respect to a signal represented by the input digital angle-modulated signal;
a digital oscillator (84, 85) which inputs a control signal (xcfx89(n)) designating an oscillation frequency thereto and generates a digital internal oscillation signal (II) representing a signal having an oscillation frequency designated by the control signal and a second ninety-degree phase shifted signal (IQ) which represents a signal whose phase is substantially ninety degrees out of phase with respect to the signal represented by the digital internal oscillation signal;
an arithmetic unit (82) which inputs the digital angle-modulated signal (SI), the first ninety-degree phase sifted signal (SQ), the digital internal oscillation signal (II) and the second ninety-degree phase shifted signal (IQ), and computes a difference between a product of the digital angle-modulated signal (SI) and the second ninety-degree phase shifted signal (IQ) and a product of the digital internal oscillation signal (II) and the first ninety-degree phase shifted signal (SQ) to generate a difference signal (xcex5(n)) which represents the difference; and
a frequency control unit (83) which supplies to the digital oscillator (84, 85) the control signal (xcfx89(n)) designating an oscillation frequency by which the difference signal from the arithmetic unit (82) indicates substantial zero, based on the difference signal (xcex5(n)), and outputs the control signal as a digital demodulated signal (xcfx89(n)).
According to this angle demodulator, an oscillation frequency of the digital internal oscillation signal approaches one value determined in accordance with the value of the digital angle modulated signal, independent of the degrees of the phase shifting of the digital angle modulated signal. Even if the phase of the digital angle modulated signal shifts greater than 2xcfx80 radians, the digital angle modulated signal is appropriately demodulated.
The phase shifter (81) may be, for example, a Hilbert transformer.
The frequency control unit (83) outputs the control signal designating as an oscillation frequency a value xcfx89(n) expressed by equation 2 where both Kp and Ki represent predetermined constants, xcexa3xcex5 represents a sum of the values of the supplied difference signals, and xcex5(n) represents the value of n-th supplied difference signal. In this case, for example, the digital oscillator (84, 85) inputs the control signal and generates the digital internal oscillation signal whose phase is substantially equal to a value xcex8(n) expressed by equation 3 when fs represents a sampling frequency of the digital angle modulated signal.
xcfx89(n)=xcfx89(nxe2x88x921)+Kpxc2x7xcex5(n)+Klxc2x7(1/fs)xc2x7xcexa3xcex5xe2x80x83xe2x80x83(2)
xcex8(n)=xcex8(nxe2x88x921)+(1/fs)xc2x7xcfx89(nxe2x88x921)xe2x80x83xe2x80x83(3)
The digital oscillators may comprises:
a digital integrator (84) which inputs the digital demodulated signal (xcfx89) thereto and generates a digital integration signal (xcex8) which represents a result of the substantial integration of the digital demodulation signal; and
a converter (85) which generates the second ninety-degree phase shifted signal and the digital internal oscillation signal having a phase represented by the digital integration signal.
The angle demodulator may comprises:
an A/D converter (6, 7) for inputting an analog angle-modulated signal and converting the analog angle-modulated signal into the digital angle-modulated signal to be output; and
a D/A converter (9) for inputting the digital demodulated signal from the frequency control unit and for converting the digital demodulation signal into an analog signal to be outputted.
As having these features, the angle-demodulator can demodulate an analog angle-modulated signal and give analog reproduction.
According to the second aspect of the present invention, there is provided an angle demodulator comprising:
a phase shifter (81) for inputting an angle modulated signal (SI) and for generating a first ninety-degree phase shifted signal (SQ) which is substantially ninety degrees out of phase with respect to the angle modulated signal (SI);
an internal oscillator (84, 85) for inputting a control signal (xcfx89) designating an oscillation frequency and for generating an internal oscillation signal (II) having the oscillation frequency designated by the control signal and a second ninety-degree phase shifted signal (IQ) which is substantially ninety degrees out of phase with respect to the internal oscillation signal;
an arithmetic unit (82) for generating a difference signal (xcex5) representing a difference between a product of the angle modulated signal and the second ninety-degree phase shifted signal and a product of the internal oscillation signal and the first ninety-degree phase shifted signal; and
a frequency control unit (83) for supplying to the internal oscillator the control signal (xcfx89) by which the difference signal (xcex5) indicates zero and for outputting the control signal as a demodulation signal.
The oscillation frequency of the internal oscillation signal approaches one value determined by the value of the angle modulated signal, independent of the degree of the phase shifting of the angle modulated signal. Even if the phase of the angle modulated signal shifts greater than 2xcfx80 radians, the angle modulated signal is appropriately demodulated.
The phase shifter (81) may be a Hilbert transformer or the like, for performing a Hilbert transform on the angle-modulated signal and for generating the first ninety-degree phase shifted signal.
The frequency control unit (83) inputs the difference signal (xcex5) and outputs the control signal by which the oscillation frequency of the internal oscillator is varied so as to be substantially proportional to a sum of a value proportional to the difference signal and an integration value of the difference signal.
The internal oscillator may comprise:
an integrator (84) which integrates the demodulated signal and generates an integration signal; and
a converter (85) which generates the internal oscillation signal (II) having a value designated by the integration signal as a phase, and the second ninety-degree phase shifted signal (IQ) which is substantially ninety degrees out of phase with respect to the internal oscillation signal.
Each of the signals may be an analog signal or a digital signal whose value represents a substantial instantaneous value of each of the signals.
According to the third aspect of the present invention, there is provided an angle-demodulator comprising:
an oscillator (84, 85) which inputs a control signal (xcfx89) designating an oscillation frequency and generates an internal oscillation signal (II) having the oscillation frequency designated by the control signal;
an arithmetic unit (82) inputs an angle-modulated signal (SI) and the internal oscillation signal (II) and generates a phase difference signal (xcex5) representing a phase difference between the angle-modulated signal (SI) and the internal oscillation signal (II); and
a frequency control unit (83) which generates the control signal controlling an oscillation frequency of the internal oscillation signal (II) so as the phase difference signal to approach zero on the basis of the phase difference signal (xcex5), supplies the control signal to the internal oscillator and outputs the control signal as a demodulated signal.
According to the angle-demodulator of the present invention, the oscillation frequency of the digital internal oscillation signal approaches one value determined by the value of the angle modulated signal, independent of the degree of the phase shifting of the digital angle-modulated signal. Therefore, even if the phase of the digital angle modulated signal shifts greater than 2xcfx80 radians, the digital angle modulated signal is appropriately demodulated.
Each of the signals may be an analog signal or a digital signal whose value represents a substantial instantaneous value of each of the signal.
According to the fourth aspect of the present invention, there is provided a method for demodulating an angle modulated signal comprising:
an oscillating (84, 85) step of inputting a control signal designating an oscillation frequency thereto and generating an oscillation signal having the oscillation frequency designated by the control signal;
an arithmetic step (82) of inputting an angle-modulated signal (SI) and the oscillation signal (II) thereto, and generating a phase difference signal (xcex5) representing a phase difference between the angle modulated signal and the oscillation signal; and
a frequency control step of generating the control signal which controls the oscillation frequency of the internal oscillation signal so as the phase difference signal to approach zero, on the basis of the phase difference signal, and outputting the control signal as a demodulated signal.
The oscillating step may comprise the steps of inputting the control signal (xcfx89) designating the oscillation frequency, and generating the oscillation signal (II) having the oscillation frequency designated by the control signal and a second ninety-degree phase shifted signal (IQ) which is substantially ninety degrees out of phase with respect to the oscillation signal,
the arithmetic step may comprise the steps of inputting an angle-modulated signal (SI) and generating a first ninety-degree phase shifted signal (SQ) which is substantially ninety degrees out of phase with respect to the angle modulated signal (SI), and generating a difference signal (xcex5) representing a difference between a product of the angle-modulated signal and the second ninety-degree phase shifted signal and a product of the oscillation signal and the first ninety-degree phase shifted signal, and
the frequency control step may comprise the steps of generating the control signal (xcfx89) so as to control the difference signal (xcex5) indicating zero, and outputting the control signal as a demodulated signal.
The phase shifting step converts the angle-modulated signal by means of a Hilbert transform to generate the first ninety-degree phase shifted signal (SQ).
The frequency control step inputs the difference signal and controls the oscillation frequency so as to be substantially proportional to a sum of a value proportional to the difference signal and an integration value of the difference signal.
The oscillation step further comprises, for example, the steps of integrating the control signal so as to generate an integration signal; and generating the oscillation signal (II) having a phase designated by the integration signal and the second ninety-degree phase shifted signal (IQ) which is substantially ninety degrees out of phase with respect to the oscillation signal.
The frequency control step outputs the control signal designating a value xcfx89 (n) obtained by equation 4 as an oscillation frequency, when each of Kp and Ki represents a predetermined constant, xcexa3xcex5 represents a total sum of values of the supplied difference signals, a value xcex5(n) represents the value of the n-th supplied difference signal, and
the oscillation step may input the control signal and generate the oscillation signal whose phase is substantially equal to a value xcex8 (n) expressed by equation 5, when fs represents a sampling frequency of the angle modulated signal.
xcfx89(n)=xcfx89(nxe2x88x921)+Kpxc2x7xcex5(n)+Kixc2x7(1/fs)xc2x7xcexa3xcex5xe2x80x83xe2x80x83(4)
xcex8(n)=xcex8(nxe2x88x921)+(1/fs)xc2x7xcfx89(nxe2x88x921)xe2x80x83xe2x80x83(5)
Each of the signals may be an analog signal or a digital signal whose value indicates a substantial instantaneous value of the signal.